P. Veillet

Magnetic properties of ferromagnetic thin sandwiches correlated to their crystallographic structure and interface morphology: HREM and NMR study

Abstract High resolution electron microscopy (HREM) studies were made on Au/Co/Au sandwiches having ultrathin Co layers. These films were shown previously to possess an important interface anisotropy. We can conclude from the present experiments that there are some fluctuations of the c axis around the film normal, and change in lattice parameter for certain crystallites. (The Co layer is composed of small crystallites of about 7 nm.) All these crystallographic characteristics explain why the Co NMR spectrum is asymmetric with a low frequency tail.

Transport properties and magnetic behaviour of La1-xSrxMnO3 single crystals

A semiconductor-metal transition occurring just below the Curie temperature TC was observed in single crystals of perovskite La1-xSrxMnO3 for x>or=0.175. For x or=0.13 whereas huge MR persists well below TC for the x=0.10 sample, which is probably due to a non-collinear spin configuration.

Conductivity and magnetoresistance of La1−xSrxMnO3 and La1−xSrxMn1−yMgyO3 single crystals

Abstract We present conductivity and MR measurements on single crystals of the mixed perovskite manganites La 1− x Sr x MnO 3 (0.1 x x = 0.17 and a metallic one at higher Sr concentrations. The MR is maximum near T c , which is strongly concentration dependent. In the sample with x = 0.15, the MR is not saturated even at 20 T. For the same composition, an important anisotropy of the electrical conductivity with respect to the [001] direction appears at T c and increases at lower temperatures. In the compound La 0.825 Sr 0.175 Mn 0.94 Mg 0.06 O 3 , the substitution of Mg for Mn lowers T c , in contrast to what is observed by substituting Sr for La. The crystal exhibits a well defined first order transition from orthorhombic to rhombohedral structure at a temperature T s which can be shifted by a magnetic field.

Giant magnetoresistance and super-paramagnetism in Co/Au multilayers

Abstract We present results for the magnetoresistance and magnetisation of MBE grown Co/Au multilayers with very thin Co thicknesses. These multilayers exhibit superparamagnetic properties similar to the non-multilayer granular systems when the Co layer is discontinuous. By controlling the growth rate, it was found that we can vary the Co island size and therefore the blocking temperature of superparamagnetic particles.

Cu NMR study of a single crystal of the spin-Peierls compound CuGeO3

Abstract We have measured the nuclear quadrupole resonance (NQR) and the nuclear magnetic resonance (NMR) of copper nuclei in a single crystal of CuGeO 3 , at 4K. The NQR frequency is practically independent of temperature whereas the spin-lattice relaxation time T 1 increases drastically as the temperature decreases. This behaviour of T 1 can be qualitatively understood by means of the fluctuation-dissipation theorem, which relates the imaginary part of the electronic susceptibility to the nuclear relaxation. The absence of a static Cu 2+ hyperfine field indicates that there is no long-range order. NMR was used to investigate the local symmetry of the Cu sites. The local symmetry axes, x, y, z were then determined. The asymmetry parameter of the electric field gradient was found to be 0.16. The principal values of the Cu 2+ g tensor were deduced as g x = 2.070, g y = 2.004 and g z = 2.407.

Spin-glass-like behaviour of the mixed spinel systems MnxZn1-xCr2O4 and Mn0.75Mg0.25(Cr1-yVy)2O4

Abstract NMR and susceptibility measurements have been made on a randomly mixed insulating ferrimagnet and antiferromagnet, MnxZn1-xCr2O4. The thermoremanence and the induced unidirectional anisotropy were observed for concentrations lower than x = 0.80, after field cooling. The compound Mn0.75Mg0.25Cr2O4 shows similar behaviour. When the latter is doped with V3+ at the B sites, its magnetic anisotropy increases strongly, but the change in the unidirectional anisotropy is smooth.

NMR study in a single crystal of the one-dimensional antiferromagnet CsCuCl3

Abstract The Cu 2+ hyperfine field directions were determined by means of the 63 Cu magnetic resonance on a single crystal of CsCuCl 3 in a dc magnetic field. They make an angle 57° with the c axis. It was suggested a possible spiral spin configuration with the Cu 2+ spins lying in the c plane. From the temperature dependence of the nuclear resonance frequency it was deduced an energy gap of about 4°K for this antiferromagnetic compound.

NMR and magnetization studies of the mixed spinel ferrites GaxFe1-xNiCr1-yAlyO4

Abstract The hyperfine fields at the nuclei of Cr3+ and Ga3+ ions in the spinel systems GaxFe1-xNiCrO4 and Ga0.4Fe0.6NiCr1-yAlyO4 have been measured by the spin-echo method. The caning angle at the B-site deduced from NMR and magnetization data is found to increase with both increasing x and increasing y.

Hyperfine interactions in Zn-doped K2CuF4

Abstract The hyperfine fields of Cu 2+ and F - ions nearest to a Zn 2+ ion are measured by a spin echo method in a single crystal of K 2 Cu 0.95 Zn 0.05 F 4 . It is shown that the local symmetry due to the Jahn-Teller distortion is practically unaffected by the proximity of Zn 2+ ions.